Your vehicle does need to be grounded when you tighten all the nuts and bolts associated with the shock replacement. You could try setting the rear of the truck on cinder blocks, assuming you don't have access to any type of ramp system, and that will give you enough height to get under and tighten everything. As for the rear sway bar, 2 of each:
http://www.trademotion.com/partlocator/index.cfm?action=getLocator&siteid=213951&chapter=ERI5768&appSectionid=2788&groupid=51856&subgroupid=2544&componentid=0&make=67&model=M%20class&year=1999&graphicID=5768500&callout=3&catalogid=2
I did mine along with the shocks and had positive results. I'm not saying that is your problem though; good luck.